This invention relates generally to a measure while drilling apparatus used for directional drilling of boreholes in the earth. Such drilling generally includes a drilling motor, sensors for drill string or hole inclination, direction and various drilling parameters as near to the bit as feasible means for transmitting such sensor data to a location above the drilling motor, and a bent subassembly which provides the means for directional drilling.
In the prior art, drilling motors are known often to be of the “Moineau” or progressive-cavity type, operated by the flow of drilling fluids pumped down through the drill string from the surface. Such motors require coupling mechanism to couple the eccentric motion of the drill-motor rotor to the drill bit. One well-known coupling means is a flexible torsion bar as disclosed in certain prior patents, such as U.S. Pat. Nos. 5,090,497, 5,456,106 and 5,725,062. In the prior art it is also known to use sensing means for inclination, direction and other drilling parameters below the drilling motor. In U.S. Pat. No. 5,456,106 a sensor housing having an axial opening therethrough provides a pathway for the flow of the drilling fluid to the bit region. Since the sensing means are below the drilling motor, some way for short range communication is required to transmit the data to a point above the drilling motor for retransmission to the surface. Wired connections for such short range transmission are shown in U.S. Pat. No. 5,456,106 in which the wires are placed within the outer case of the drilling motor and in U.S. Pat. No. 5,725,062 wherein the wires are placed within the rotor of the drilling motor. U.S. Pat. No. 5,160,925 discloses use of a toroid core with a primary winding on the core, the drill string being located through the center of the opening. Electrical signal applied to the primary winding induce currents in the drill string components, that are detected by a similar toroid core at a higher location in the drill string above the drilling motor. U.S. Pat. No. 6,057,784 discloses a solenoid transmitting core with ferrite elements embedded in the core to enhance the launching of a magnetic field into the drilling assembly. The wind connections disclosed in such patents have not generally been adopted. Other approaches require complicated mechanical structures and a large number of parts and assemblies.
In location of the sensing means below the drilling motor causes an increase in length of the complete assembly. Such sensing means may be a separate subassembly or may be incorporated in the bent subassembly. Incorporation in the bent subassembly may shorten the overall length, but adds to the problem of not having the inclination and direction sensors correctly aligned with the longitudinal axis of the drill string.
In the prior art, a recessed insulated conductive element is used to inject electrical signal currents directly into a formation for detection at another nearby location in the drill string using another insulated conductive element. Experiments have shown that such direct electric injection and detection of currents can provide a very simple and effective short range communication system requiring less mechanical complexity in the drill string structure below a drilling motor. Further, the reduced size of such an apparatus permits the placement of desired sensing means and data transmission means in parallel with the motor torsion bar rather than in series with the torsion bar. This provides a significant reduction in the overall apparatus. Such parallel placement of the sensing means also permits replacement in the field of the entire sensor assembly, without major disassembly of the complete measure while drilling apparatus.
This disclosure provides improved reduced-length measure while drilling apparatus by using an electric field short range data transmission apparatus and with the sensing and data transmission elements placed in parallel with the torsion bar of the drilling motor. The disclosure also provides an apparatus in which the sensing and data transmission elements can be replaced without complete disassembly of the apparatus.